A generally-used cooling system for, for example, a mold for a continuous casting is a system in which the cooling water is passed through a space formed between a recess, which is provided in the rear surface of a mold body, and a back frame. However, in this cooling system, a sufficiently high cooling efficiency cannot be obtained, and there is the possibility that the water may leak from a region between a back frame and a mold body. Moreover, it is difficult to provide this region with a seal for preventing the leakage of water therefrom. With a view to eliminating these inconveniences, a mold having a cooling water passage formed completely within a mold body has recently been proposed.
However, it is very troublesome to form such a cooling water passage in a mold of this construction. Therefore, a smaller number of larger-diameter bores is more often the case than a larger number of smaller-diameter bores. The larger the diameter of a cooling water passage, the more easily the cooling water flowing therethrough separates into supernatant, intermediate and bottom layers. This causes a great decrease in the cooling efficiency.